1. Technical Field
The present invention relates to a method of manufacturing gas generating material, and particularly to a method of making gas generating bodies from gas generating material.
2. Description of the Prior Art
A gas generating body (known as a "grain") is used to generate gas to inflate a vehicle occupant restraint such as an airbag. U.S. Pat. No. 4,696,705, assigned to the assignee of the present application, discloses such a gas generating body. The gas generating body when ignited generates nitrogen gas to inflate an airbag. The gas generating body preferably has a plurality of longitudinally extending passages parallel to the axis of the body.
Upon ignition of the gas generating body, it has a progressive rate of burn. A progressive rate of burn is one in which the burning proceeds, for a substantial part of the burn cycle, at a rate which increases. As the circumferential surfaces of the passages in the body burn, the passages widen, exposing increasingly more surface area to burning. Simultaneously, the outer circumference of the body of gas generating material shrinks, reducing the surface area exposed to burning, but this reduction in surface area is less than the increase in surface area produced by burning in the passages in the grain. At a point in the burn cycle, the burn rate ceases to increase and remains constant until near the end of the burn cycle, at which time the rate of burn decreases to zero. Nonetheless the period of increasing burn rate extends for a substantial part of the burn cycle
Typically, a body of gas generating material is manufactured by first producing a gas generating material in the form of a dry powder, flowing the dry powder into a die, and applying a high pressure to the powder so that the powder particles adhere to each other to form the body. U.S. Pat. No. 4,696,705 discloses such a process.
In order to manufacture bodies of gas generating material which have a plurality of passages to provide a progressive rate of burn, it is necessary to have a plurality of closely spaced pins which extend longitudinally in a die cavity. The gas generating material is introduced into the die cavity around the pins. If the gas generating material does not flow readily into the die cavity and around the pins, air pockets can form in the material in the die cavity. When a high pressure is applied to the material in the die cavity, the material may apply a lateral force against a pin. If an air pocket is on the side of the pin opposite the direction of action of the lateral force, the pin may break.